This invention relates to a method of making a preform suitable for use in making composite articles.
There is an increasing need for high strength polymeric materials to replace metals in many applications. The polymeric materials have the advantage of lower weight and are often less expensive and more durable than metals. Usually, however, the polymeric material is much lower in strength than the metal, and unless it is reinforced in some manner it will not meet the strength requirements for metal replacement.
Thus, polymeric composites have been developed to meet these strength requirements. These composites are characterized by having a continuous polymeric matrix in which is embedded a reinforcement material, usually a relatively rigid, high aspect ratio material such as glass fibers.
These composites usually are molded into a predetermined shape, which is in many cases asymmetric. In order to get the reinforcement material into the composite, the reinforcement material is usually placed into the mold in a first step, followed by closing the mold and then introducing a fluid molding resin. The molding resin fills the mold, including the interstices between the fibers, and hardens (by cooling or curing) to form the desired composite. At other times, the molding resin is applied to the reinforcing fiber prior to molding. The fiber with resin is placed into a mold where temperature and pressure are applied, curing the resin to prepare the desired composite.
It is highly advantageous that the reinforcement material is uniformly distributed throughout the composite, or else the composite will have weak spots where the reinforcement is lacking. Thus, the reinforcement material is desirably prepared to ensure that the individual fibers are distributed evenly throughout the composite. In addition, the individual fibers desirably resist flowing with the molding resin as it enters the mold.
For these reasons, it is conventional for the reinforcement to be formed into a mat outside of the mold, and the preformed mat is then placed in the mold and either impregnated with resin in order to make the final composite article, or simply heated and pressed to make a very low density composite article. The mat is generally prepared by forming the reinforcing fibers into a shape matching the inside of the mold and applying a binder to the fibers. In some instances a thermosetting binder is pre-applied, and then cured after the fibers are shaped into a mat. In other methods, a thermoplastic binder is applied, so that in a subsequent operation the binder can be heated and softened and the mat subsequently shaped. This binder "glues" the individual fibers to each other so that the resulting mat retains its shape when it is transferred to the mold for further processing. The binder also helps the individual fibers retain their positions when the fluid molding resin is introduced into the mold. In some cases, a molding resin can alternatively be applied to the reinforcing fiber prior to molding. The fiber with binder and resin is placed into a mold where temperature and pressure are then applied, curing the resin to prepare the desired composite.
The binders used heretofore have been primarily of three types. Unfortunately, the conventional use of each of these types of binders has significant drawbacks. The predominantly used binders have been solvent-borne polymers, i.e., liquids, such as epoxy and polyester resins. The solvent-borne binders are usually sprayed onto the mat via an "air-directed" method, and then the mat is heated to volatilize the solvent and, if necessary, cure the binder. This means that the application of binder is at least a two-step process, which is not desirable from an economic standpoint, and also that the use of solvents is encountered, which raises environmental, exposure and recovery issues. Dealing with these issues potentially adds significantly to the expense of the process. The procedure is also energy-intensive, as the entire mat must be heated just to flash off solvent and cure the binder. The curing step also makes the process take longer. Finally, use of these binders is also extremely messy, with high maintenance costs associated with keeping the work area and the screen itself clean. In this case, where the binder may be a low viscosity fluid, it tends to flow over and coat a large portion of the surface of the fibers. When a composite article is then prepared from a preform made in this way, the binder often interferes with the adhesion between the fibers and the continuous polymer phase, to the detriment of the physical properties of the final composite.
Powdered binders have also been used. These can be mixed with the fibers and then the mass formed into a preform shape, which is then heated to cure the binder in situ. Alternatively, these binders can be sprayed to contact the fibers, but simple substitution of a powdered binder in this air-directed method raises its own problems. The powdered binders cannot be applied unless a veil is first applied to the screen to prevent the binder particles from being sucked through. Again, this adds to the overall cost and adds a step to the process. Airborne powders also present a health and explosion hazard. Finally, the use of powdered binders also requires a heating step to melt the binder particles after they are applied to the fibers, which renders this process energy-intensive as well.
A third type is heated thermoplastic materials, which can be melted and sprayed as a binder. Use of these materials makes any subsequent heating step unnecessary, since the binder does not require it to achieve some undetermined measure of adhesion to the fibers. However, in this method, "lofting", or inadequate compaction of the preform, typically occurs, because the thermoplastics are conventionally heated to any random temperature above their melting points, leading to a lack of uniformity in their cooling patterns and extensive migration along fiber surfaces, which in turn allows some of the fibers to "bounce back" before they are set into place by the solidifying thermoplastic. This may result in formation of a lower density preform than desired, density gradients throughout the preform, and poor adhesion of the fibers to each other.
In view of the problems discussed hereinabove, it would be desirable to provide a simpler method for making preforms in which the problems associated with using solvent-borne, powdered or thermoplastic binders are minimized or overcome.